Prove behavior with deterministic tests

Add executable validation at the trust boundaries of RAG API with complete green/v1 and blue/v2 stacks behind one router. Reject unknown fields where they matter, bound string sizes, and coerce only after auth/signature checks when raw bytes are security-relevant. Invariant under test: blue must pass smoke + /readyz before cut; old green kept warm for 30m rollback. A TypeScript type or Python annotation is not runtime validation — pair them with parsers.

Automate the fixtures from setup, including a recreation of BG-SCHEMA-ONESIDE-11. Assert both the visible error and the absence of side effects (no provider call, no queue write, no flag flip). Where metrics matter, assert label enums stay bounded.

Document which failures are client mistakes (4xx) versus operator/config mistakes (5xx/503). Oracle still stands: cutover < 30s; rollback restores green with zero dual-write divergence. Validation should make accidental “success with empty body” impossible for release engineer flipping 100% router cut after green smoke.

Property ideas: shuffled field order, Unicode edges, maximum-length strings, and replayed timestamps. Where money, identity, or citations matter, assertion messages should cite the field name. Do not snapshot entire provider payloads in tests; assert semantically. If validation fails open “to keep the demo working,” you have inverted the lab. Tie at least one CI job to the BG-SCHEMA-ONESIDE-11 fixture so main cannot regress silently. Re-read blue must pass smoke + /readyz before cut; old green kept warm for 30m rollback after each new parser — convenience helpers love to bypass it.

Table-drive status codes and error codes so reviewers see coverage at a glance. Include a Unicode normalization case if user text is accepted. Verify that oversized bodies fail before CPU-heavy work. Where digests or versions are pinned, assert mismatch behavior. Keep golden files small enough to read in review. CI should fail on skipped tests that mark the incident fixture as xfail without a ticket link. In this chapter the product is RAG API with complete green/v1 and blue/v2 stacks behind one router, the human stakeholder is release engineer flipping 100% router cut after green smoke, and the incident id you design against is BG-SCHEMA-ONESIDE-11. Re-state the oracle in your notes — cutover < 30s; rollback restores green with zero dual-write divergence — and keep the invariant visible: blue must pass smoke + /readyz before cut; old green kept warm for 30m rollback. Track cutover_seconds and postcut_error_rate as the scoreboard. Surface under change control: router → green|blue /v1/rag/answer. If you only have forty minutes, finish the fixture for schema migration applied only on blue — green rollback serves 500s before polishing UI. Promotion language stays ternary: promote, hold, or roll back based on evidence, not hope.